When the clasp is opened, you can see a little arrow shaped opening in the middle part. You can flick that part up by inserting a small, flat object in that opening. After it is open, you can move that part of the clasp up or down the strap. Please be aware of the little indentations on the back of the strap. Once you have reached the correct spot, all you have to do is click that part of the clasp shut.
This video shows the entire process.